The maintenance of proper phase correlation among such oscillators, particularly in a communication system, is important for the purpose of avoiding loss of information, cross-talk, desynchronization and interruption of service.
In principle, the several oscillators could be kept in step either by a common external reference oscillation applied to their synchronizing inputs or by a cyclical connection in which the output signal of any oscillator serves as a synchronizing signal for the immediately following oscillator. In the first instance, a control circuit associated with each oscillator detects frequency or phase disparities between the generated oscillations and the reference wave and, if these disparities exceed a certain limit, causes the automatic replacement of the faulty oscillator by a properly operating one. In the second instance, a fault occurring in one oscillator also affects the next-following oscillator which may then be needlessly eliminated from the chain.